U.S. patent application number 12/152789 was filed with the patent office on 2009-11-19 for rf antenna assembly having an antenna with transversal magnetic field for generation of inductively coupled plasma.
Invention is credited to Yuri Glukhoy, Tatiana Kerzhner, Anna Ryaboy.
Application Number | 20090284421 12/152789 |
Document ID | / |
Family ID | 41315667 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090284421 |
Kind Code |
A1 |
Glukhoy; Yuri ; et
al. |
November 19, 2009 |
RF antenna assembly having an antenna with transversal magnetic
field for generation of inductively coupled plasma
Abstract
An antenna assembly that consists of a holder which supports a
transversal RF antenna with a plurality of multiturn coils
connected in series or in parallel and intended for generation of
an inductively coupled plasma discharge inside a container with a
high plasma density in vicinity of the container's inner walls. The
aforementioned discharge is used for inducing in the container a
plasma chemical reaction between oxygen and organosilane with
resulting deposition of the reaction product in the form of silicon
dioxide onto the inner walls of the container for forming a
fluid-impermeable barrier layer. A specific feature of the antenna
is that it generates a magnetic field transversal to the
longitudinal axis of the antenna, i.e., normal to the container's
walls, where a maximal electric field, maximal plasma density and,
correspondingly, maximal rate of deposition of silicon dioxide on
the wall are provided.
Inventors: |
Glukhoy; Yuri; (San
Francisco, CA) ; Kerzhner; Tatiana; (San Francisco,
CA) ; Ryaboy; Anna; (San Francisco, CA) |
Correspondence
Address: |
Tatiana Kerzhner
1927 - 31st. Avenue
San Francisco
CA
94116
US
|
Family ID: |
41315667 |
Appl. No.: |
12/152789 |
Filed: |
May 16, 2008 |
Current U.S.
Class: |
343/701 |
Current CPC
Class: |
H01Q 7/00 20130101; H01Q
1/26 20130101 |
Class at
Publication: |
343/701 |
International
Class: |
H01Q 1/26 20060101
H01Q001/26 |
Claims
1. An antenna assembly insertable into a container having inner
walls and a longitudinal axis, the antenna assembly comprising: a
transversal RF antenna having means for creating a magnetic field
transversal to the longitudinal axis of the container and for
generating an inductively coupled plasma discharge inside the
container with a high plasma density in vicinity of the inner
walls; a container holder that supports the aforementioned
transversal antenna in the direction of the longitudinal axis of
the container; a precursor gas supply tube inserted into the
container; and at least one opening in the antenna holder for
evacuation of the container.
2. The antenna assembly of claim 1, wherein the antenna holder is a
cap for sealingly closing the container.
3. The antenna assembly of claim 1, wherein the transversal RF
antenna has a shape that conforms to the shape of the inner walls
of the container.
4. The antenna assembly of claim 2, wherein the transversal RF
antenna has a shape that conforms to the shape of the inner walls
of the container.
5. The antenna assembly of claim 1, wherein the means for creating
a magnetic field transversal to the longitudinal axis of the
container comprise at least two multitum coils connected in series
or in parallel.
6. The antenna assembly of claim 4, wherein the means for creating
a magnetic field transversal to the longitudinal axis of the
container comprise at least two multitum coils connected in series
or in parallel.
7. The antenna assembly of claim 5, wherein the multiturn coils
have turns with diameters that increase outward in the
aforementioned transverse direction and the distances between the
adjacent turns exceed a threshold of breakdown.
8. The antenna assembly of claim 5, wherein the turns may have a
shape selected from a round, rectangular, tapered, or nontapered
configurations.
9. The antenna assembly of claim 5, wherein the coils are wound
from wires or tubes for cooling medium.
10. The antenna assembly of claim 9, wherein coils are made from
copper.
11. The antenna assembly of claim 7, wherein the coils are wound
from wires or tubes for cooling medium.
12. The antenna assembly of claim 11, wherein coils are made from
copper.
13. The antenna assembly of claim 8, wherein the coils are wound
from wires or tubes.
14. The antenna assembly of claim 13, wherein coils are made from
copper.
15. The antenna assembly of claim 1, wherein the transversal RF
antenna is further provided with a solenoid that together with an
inert gas supplied into the container constitutes a discharge
plasma ignition trigger for initial ignition of plasma in the
container.
16. The antenna assembly of claim 4, wherein the transversal RF
antenna is further provided with a solenoid that together with an
inert gas supplied into the container constitutes a discharge
plasma ignition trigger for initial ignition of plasma in the
container.
17. The antenna assembly of claim 1, where the precursor gas supply
tube has a plurality of gas-distribution openings.
18. The antenna assembly of claim 17, wherein distribution of the
precursor gas supply openings is non-uniform for providing instant
uniformity of flow of the precursor gas in the container.
19. The antenna assembly of claim 5, where the precursor gas supply
tube has a plurality of gas-distribution openings.
20. The antenna assembly of claim 19, wherein distribution of the
precursor gas supply openings is non-uniform for providing instant
uniformity of flow of the precursor gas in the container.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to plasma processing
and in particular to an RF antenna assembly having an antenna that
generates a transversal magnetic field. More specifically, the
invention relates to an antenna assembly having an RF antenna with
a transversal magnetic field for generation of inductively coupled
plasma. The antenna assembly of the invention is for use in an
apparatus for plasma-enhanced chemical vapor deposition (PECVD) of
thin films, especially onto the interior surfaces of hollow
containers such as bottles, etc., as well as for combined cleaning,
barrier coating, and sterilizing of food containers or
pharmaceutical packaging materials.
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART
[0002] Although glass containers are substantially impenetrable and
provide products with long shelf life, they are heavy and expensive
for manufacturing and transportation. Containers made of polymeric
materials now replace glass containers in applications where
traditionally glass containers were used. Plastic containers are
less susceptible to breakage, less expensive to manufacture, and
lighter and less expensive to ship. Used for packaging are plastic
materials such as polyethylene terephthalate (PET) and high-density
polyethylene (HDPE) in the form of bottles or other shapes having
an opening at the top.
[0003] PET containers are used for liquids such as wine, soft
drinks, etc. HDPE is used for packaging milk, water, juice,
cosmetics, shampoo, etc. HDPE containers are more available for
recycling than PET containers and serve a second life for the
packaging of liquid laundry detergents, shampoo, conditioner, motor
oil, etc.
[0004] However, glass properties such as chemical resistance and
permeability are not attainable for plastics. Polymer-chain
clearance of a plastic structure is less than 1 nm and, hence,
cannot prevent penetration of low-molecular gases having molecules
ranging in size from 0.3 to 0.4 nm. The walls of such packages are
permeable in both directions in relation to gases such as oxygen,
carbon dioxide, etc. The shelf life of liquids is limited,
especially for soft drinks and other CO.sub.2-containing liquids. A
long shelf life is required for carbonated beverages (soft drinks
and beer), fruit juice, cosmetics, chemicals, and pharmaceuticals.
Deterioration of liquids on the shelf, especially in hot weather,
is caused by several factors: (1) oxygen, which is responsible for
entering the container through the plastic wall from outside; (2)
carbon dioxide, which escapes through the same container wall; and
(3) low chemical resistance of a PET container to strong contents
such as a carbonated soft drink or alcoholic beverages. Molecules
of liquid absorbed at high temperature in hot weather or during
microwave heating are combined with hydroxyl, thus grouping the
polymer matrix and weakening the existing hydrogen bounds between
the polymer molecules. As a result, interchange distances increase
and create free volume, which facilitates the diffusion of oxygen
and perhaps the diffusion of other gases as well. If a PET package
contains a flavor compound (such as orange juice or apple juice),
this compound causes swelling of the PET container, i.e., opening
the structure and further increasing the specific free volume that
leads to oxygen transport. Therefore, flavor absorption
significantly increases oxygen permeability of PET.
[0005] One can expect a reduction in shelf life of oxygen-sensitive
products because of higher oxygen permeability. At 23.degree. C.,
50% relative humidity (RH), and atmospheric pressure (oxygen at
0.21 atmosphere) outside a bottle, a 0.5-liter nominal-volume
bottle formed from PET has an oxygen transmission rate of 0.126
cc/bottle/day. Under the same conditions, the same bottle formed
from HDPE has an oxygen transmission rate of 8.47 cc/bottle/day.
Flavoring ingredients of low-molecular organic compounds existing
in drinks such as lemonade absorb the plastic material and thus
deteriorate the quality of the drink. For these reasons, plastic
containers are unsuitable for drinks, especially those with carbon
dioxide, alcohol, and flavoring ingredients.
[0006] In order to prolong the shelf life of these liquid products,
a better gas barrier is required. The barrier property of plastic
containers can be improved by coating the inner walls of these
containers with a transparent layer, e.g., quartz-like SiO.sub.2.
The aforementioned barrier layer should remain after hot filling or
pasteurization. Besides reducing the permeability of the
containers, the layer that absorbs UV irradiation, which causes
deterioration in the taste of wines and other beverages, is also
included in this barrier. Such coating is provided by means of
plasma-enhanced chemical gas-phase deposition (PECVD) of an
organosilicon compound having an excess of oxygen.
[0007] The PECVD process is described, e.g., by J. Felts in U.S.
Pat. No. 6,180,191 issued in 2001. A PECVD-applied silicon dioxide
(SiO.sub.2) layer on the inside surface of a PET bottle prevents
the ingress of oxygen and the egress of carbon dioxide that would
affect the taste of the product and its shelf life. After
deposition of a thin silicon oxide coating, the oxygen transmission
rate is reduced to 0.076 cc/bottle/day.
[0008] The PECVD process first deposits a transparent adhesive
layer of nanocrystalline SiO and then a colorless silicon oxide
(SiO.sub.x) barrier layer having a thickness of 0.01 to 0.1 micron.
The SiO.sub.x layer improves the oxygen-barrier properties of a
bottle more than 10 times, and the SiO.sub.2 barrier, specifically,
improves this property more than seven times. These barrier
improvements remain after hot filling or pasteurization. In
addition to the use of PECVD in the food and pharmaceutical
industries, application of a PECVD barrier onto the inner surfaces
of hollow objects may be used in automotive and piping industries
wherein plastic materials such as HDPE are used to replace metals
because of their excellent tensile strength and impact properties
at temperatures as low as -50.degree. C. and at temperatures as
high as 70.degree. C., which match the temperature range in fuel
tanks and pipe lines. Since HDPE is low in weight and cost, it is
competitive with steel. However, HDPE has one drawback, and this is
permeation of fuels. In order to overcome this drawback, it is
necessary to develop an improved barrier coating suitable for
application onto the inner surfaces of HDPE tanks and pipes,
especially those designed to contain gasoline, alcohol, or other
toxic, corrosive, and health-hazardous materials. Moreover, the
same coating system is supposed to serve as an inductive probe to
provide quality control of the thickness, uniformity, and integrity
of the barrier in the inner surface of the wall after the
deposition process. The SiO.sub.2 coating has high optical
transparency and a markedly improved barrier effect as well as
greater tensile strength. Silicon dioxide is nontoxic and does not
affect the recycling of PET and HDPE.
[0009] The inner container coating of SiO.sub.2 provides an
excellent gas permeation barrier because of two important
properties. First, the coating on the interior surface of the
container is not subject to abrasion during shipment and handling
when compared to the exterior surface of the container. Second, by
forming the coating on the interior surface of the container,
degradation of the product within the container from direct
interactions between the product and the container is
prevented.
[0010] Thus, there is a demand for a simple, inexpensive, and
reliable process for application of barrier coatings onto the inner
walls of polymeric containers. The process should have a fast cycle
time to accommodate production demands and be suitable for
integration into a bottle-molding production line, such as a Husky
molding system with throughput of 15,500 bottles per hour. Further,
the barrier coating should have good uniformity, and the
barrier-coated polymeric container should be easy to recycle.
[0011] A plasma-enhanced chemical vapor deposition (PECVD) coating
from a gaseous phase is well known and is used in the semiconductor
industry to treat semiconductor wafers. However, a flat substrate
such as a semiconductor wafer, which is an object of deposition,
can be treated at high temperatures with application of a bias
voltage, while in the case of plastic containers, the material of
such containers has a low melting point that cannot withstand high
temperatures. Plasma discharge is developed by an RF antenna
introduced into the container together with a gas mixture and when
the RF antenna is energized, this causes a plasma-chemical reaction
that results in generation of silicon dioxide, which is deposited
onto the inner walls of the containers in the form of a thin
barrier layer of SiO.sub.2. The plasma-chemical reaction can be
conducted between different silicon-containing gases such as silane
or disilane and oxygen-containing gases such as nitrogen dioxide,
nitrous oxide, etc. Because of the flammability and explosiveness
of silanes, the above process requires special, expensive
facilities in the semiconductor industry. The food industry prefers
to conduct the processes under less expensive, unpretentious
conditions with a safer organosilicon or siloxane and by conducting
the plasma chemical-reaction with pure oxygen. The plasma-chemical
reaction may also have safe-reaction byproducts, such as CO.sub.2
and water. Plasma discharge inside a container decomposes siloxane
vapor and breaks off methyl groups. Further, the oxygen oxidizes
the condensable siloxane backbone (Si--O--Si) resulting from the
organosilicon decomposition, thereby forming a plasma-enhanced
chemical vapor deposition (PECVD) thin film of silicon oxide
(SiO.sub.x) on the interior surface of the container. Gaseous
organosilicon is received, for example, from liquid
tetraethylorthosilicate (TEOS). TEOS can be converted into vapor by
using a direct liquid injection subsystem DL125-C (a product of MKS
Company) that includes a vaporizer that evaporates the liquid into
vapor for introducing it into the processing system. Byproducts
(CO.sub.2 and water) are removed by means of a vacuum system
through small holes provided in a bottle holder.
[0012] The pure SiO.sub.2 barrier, however, presents some
disadvantages because it is brittle and can be torn during bending
and squeezing. In order to enhance durability of the coating, a
double-layer coating is preferred wherein the first thin layer is a
layer of nanocrystalline SiO.sub.2 deposited on the plastic wall.
This first layer blocks the porosity of plastic and simultaneously
improves the adhesion to plastic of the next thick layer of
amorphous SiO.sub.2 intended for contact with the liquid. This
layer increases chemical resistance of the wall to aggressive
species and simultaneously reinforces the barrier layer to prevent
rupture of the film.
[0013] The methods and devices for generation of plasma used to
form barrier coatings inside plastic containers are adopted from
the sterilization processes inside the bottles described in the
British Patent GB 1,098,693 (Menashi, et al., issued in 1968).
Menashi describes a device for sterilization inner surfaces of
plastic bottles by a method in which a central electrode is
introduced into a bottle that is surrounded by an external
electrode. Two electrodes form a coaxial system connected to a
high-frequency current source. Argon (Ar), as a process gas with
low potential of ionization, is introduced into the bottle through
a hole in the central electrode in order to develop a capacitively
coupled plasma (CCP) discharge. The device described in this patent
is characterized by a high electric field, of the order of 450
V/cm, and a very weak current, of the order of a few milliamps at
high RF power. The low current of the CCP discharge is caused by
losses of RF power sustaining the discharge because 70% of this
power is wasted by bias-current heating of the inner and outer
electrodes, as well as the plastic between the electrodes. The CCP
discharge is divided by the plastic wall on the useful discharge
inside the bottle, the discharge providing deposition and parasite
discharge between the outer electrode and the outer wall of the
bottle. The parasite discharge consumes a valuable part of RF
power. Only a small part of RF power sustains the inner discharge
used for sterilization. The treatment time of sterilization is too
long for application of this process in industry. Another
disadvantage of such a method is sputtering of the electrodes in
the CCP discharge by high-energy ions of argon and contamination of
the inner surface of the container by material of the inner
electrode.
[0014] In spite of such disadvantages, Thomas, et al (see U.S. Pat.
No. 5,378,510 for "Methods and apparatus for depositing barrier
coatings" issued in 1995) adopted the above-described geometry
because of its simplicity. The authors of the above invention
proposed to use the RF discharge to decompose process gas delivered
through a gas inlet referred to as "adjacent axis conduit extending
into hollow container. Decomposition of the process gas forms
organosilicon vapor, which is deposited in the form of a barrier
layer of SiOx onto the inner surface of the bottle, called `a
hollow polymeric container`.
[0015] RF power was applied to the outer electrode, called "an
electrically conductive shell surrounding hollow container."
[0016] U.S. Pat. No. 7,166,336 issued in 2000 to Mori, et al, and
U.S. Pat. No. 6,180,191 issued in 2001 to Felts disclose the use of
the same coaxial deposition system individually for each bottle
with some differences in bottle evacuation procedures. The Felts
process occurs in a vacuum chamber wherein the outer electrode is
located adjacent to an exterior surface of the chamber, but Mori
combines the coaxial deposition system with the vacuum chamber,
while the outer electrode serves as a wall of the vacuum chamber
that is individual for each bottle. The gas inlet in both systems
is the same as proposed by Thomas, but the supply of gas is carried
out through a plurality of small holes. The structure includes an
immersed, grounded central electrode of the coaxial system and
supplies the PECVD process with a gaseous precursor.
[0017] RF power is applied to an outer electrode located adjacent
to an exterior surface of the chamber and to the inner electrode
combined with the gas inlet. In "inverse" radial flow reactors, the
gas inlet is at the center of the lower electrode, with the gas
flow directed radially outward. The PECVD thin film, after
decomposition, deposits onto the interior surface of the container.
In the Thomas case, the bottle is rotated to enhance uniformity of
the barrier layer. In the Felts case, the inner electrode is
rotated by a magnetic drive in order to randomize the substrate
position that faces the gas stream and to optimize uniformity of
deposition. However, Mori, who reduced the clearance between the
outer electrode and the outer wall of the container in order to
reduce parasitic discharge from the bottle, has divided the outer
electrode, which tightly envelops the container, into three parts:
(1) a bottom portion of the electrode that is disposed along the
bottom of the plastic container; (2) a body portion of the
electrode that is disposed along the body of the plastic container;
and (3) a shoulder portion that is located above the body portion
enveloping the neck of the container. Resistive or capacitive
elements are interposed between the outer electrodes to provide
distribution of RF power and simultaneously to seal the outer
electrode that serves as an individual vacuum chamber. An output
terminal of the RF generator is connected only to the first portion
of the outer electrode through a matching network. The
aforementioned distribution of RF power makes it possible to
provide varying plasma density at the bottom, middle, and neck of
the container. This design provides uniformity in coating thickness
on the inner surfaces of the bottom, body, and neck of the
container, which are differently spaced from the inner electrode.
Although the devices proposed by Mori, Thomas, and Felts generate
coating films of different types (in Mori's case, these are
diamond-like films, and in the Thomas and J. Felts cases, these are
silicon dioxide films), the devices suffer from the same
disadvantages that are inherent in CCP discharge, in general.
[0018] The main disadvantage of aforementioned processes and
devices is that application of the CCP discharge to coat the inner
surfaces of a container is carried out at a low-deposition rate
limited by 10 nm/sec, a rate that significantly reduces throughput
of a production line. On the other hand, lengthy treatment of
plastic materials at high flux of thermal energy generated by
electrodes softens the plastic to the extent that after reaching a
critical point, a container can collapse. In order to increase the
deposition rate, plasma density must be increased (e.g., by
increasing pressure inside the container), and also RF power that
sustains the discharge must be increased. On one hand, increase in
pressure leads to breakdown of the space between electrodes by the
arc between both electrodes, which damages the container. On the
other hand, high RF power initiates corona discharge on the inner
electrode.
[0019] Thus, the process of coating using CCP discharge proceeds at
a very low rate and prolongs cycle time, which typically ranges
from 10 to 15 seconds. Such a low duty cycle is not suitable for
mass production of barrier-coated containers and limits throughput
to six bottles per second. Furthermore, although high-power RF
generators are expensive devices, in the case of CCP discharge they
are used with low efficiency. For example, a valuable part of RF
power is wasted for heating the outer and inner electrodes and for
a parasite discharge in the space between the outer electrode and
the outer wall of the container. A lengthy coating process can lead
to melting of the containers, taking into account that the walls
are heated by plasma. They are heated also from both sides by
infrared irradiation emitted from the overheated inner and outer
electrodes. Another problem associated with the use of CCP
discharge is bias current driven by alternating voltage through the
plastic. Such current creates additional heat, which deteriorates
and melts the structure of the plastic walls.
[0020] Another obstacle is a high surface charge on the outer and
inner surfaces of the walls that occurs between outer and inner
discharges. UV radiation from plasma initiates photoemission from
dielectric material that generates high electrical charge on the
surface, and this, in turn, causes microarcs that destroy integrity
of the thin film.
[0021] Another obstacle is a high-potential charge that remains on
the surface of the container after deposition; this charge attracts
dust, and therefore the container may require an additional
sterilization.
[0022] Evacuation of containers at a high rate by means of a vacuum
system for a quick drop in pressure is needed to create balance
between high pressure inside the container and low pressure outside
the container in order to reduce parasitic discharge, tight
enveloping of the container for reducing the space between the
outer wall of container and outer electrode with subsequent
decrease of time needed for loading the containers, heating of both
electrodes and plastic between them, and collapsing and charging of
the container walls, all of which make the CCP discharge process
highly inefficient in the formation of barrier coatings. Provision
of the outer electrode makes it impossible to apply the coating
onto the inner surfaces of plastic tanks and pipes having a
curvilinear shape.
[0023] On the other hand, known in the art is ICP discharge, which
is used as a source of light and has been used as a source of light
for some time. An ICP discharge has been described and analyzed in
literature, such as in articles by R. B. Piejack, V. A. Godyak, and
B. M. Alexandrovich titled "A simple analysis of an inductive RF
discharge," Plasma Sources Sci. Technol. 1, 1992, pages 179 to 186,
and "Electrical and Light Characteristics of RF-Inductive
Fluorescent Lamps," Journal of the Illuminating Engineering
Society, Winter 1994, pages 40 to 44. An ICP light source comprises
a vacuum vessel, an inductive coupling system immersed in the
vessel, and a high-frequency power source. In the initial stage of
operation of inductively coupled plasma, an electrical field (E
field) ionizes the fill in the gas-filled volume, and the discharge
is initially a characteristic of an E discharge. Once breakdown
occurs, however, an abrupt and visible transition to the H
discharge occurs. Inductively coupled plasma works on the principal
of producing an electric field in a body of gas by means of
electromagnetic fields induced by oscillating current in the
vicinity of the gas.
[0024] When the fields induced in the gas are strong enough, the
gas can break down and become ionized in order to generate plasma.
Such plasma has been used for a number of applications ranging from
fluorescent lighting to plasma treatment of semiconductor wafers.
During operation of an inductively coupled discharge, both E and H
discharge components are present, but the applied H discharge
component provides greater (usually much greater) power to the
plasma than the applied E discharge component.
[0025] The inductively coupled plasma has been created by either
wrapping a solenoid coil around a glass or quartz tube containing
gas ("helical induction") or by placing such a solenoid or spiral
within the volume of gas itself ("immersed induction"). In a
typical approach, an RLC circuit created by the inductive coil and
a matching circuit are tuned to resonance and develop high currents
on the coil. An alternating electromagnetic field induced within
the gas volume creates a conductive plasma discharge having
characteristics similar to secondary winding of a transformer, with
a portion of the current through the discharge being converted to
light. Lighting devices using immersed induction are described by
Hewitt in U.S. Pat. No. 966,204, issued Aug. 2, 1910. Generation of
light requires high plasma density in the center of a vessel so
that the flat spirals, or solenoids, are immersed in a vacuum bulb
having axial symmetry. However, use of axially symmetric antennas
is not applicable to elongated containers, e.g., bottles, since
they cannot generate plasma having high and uniform density near
the inner walls of containers.
[0026] An example for use of capacitively coupled plasma for
deposition of a barrier coating layer onto inner surfaces of
bottles is disclosed in German Patent DE 3,908,418, by H. Grunwald,
issued Sep. 20, 1990. This patent describes a system designed for
plasma-assisted film deposition or treatment of hollow containers
and comprises a capacitively coupled plasma system to drive a
low-pressure gas discharge within the form. Such a system also has
disadvantages, including a potentially lower deposition and
treatment rate for mass-produced applications. Similar to other
capacitively coupled plasma systems, the system of the
aforementioned invention uses high plasma sheath energies that may
result in excess heating of sensitive plastic container walls
resulting in container damage. This design is also complicated and
may require expensive and regular maintenance caused by film
deposition on power-coupling components.
[0027] Also known in the art is the use of apparatus for coating
the inner walls of containers, such as bottles, by means of
deposition from inductively coupled plasma (see, e.g., U.S. Pat.
No. 5,521,351 issued in 1996 to L. Mahoney). This invention relates
to inductively coupled plasma generated within the interior of a
hollow form held within a vacuum chamber enclosure by using a radio
frequency coil mounted within the vacuum chamber around the outer
surface of the container and closely conforming to the shape of the
hollow container. The interior of a hollow form having complex
shapes can be treated using two or more coils arranged to treat
distinct portions of the form, and the shape of the coils and the
manner in which power is supplied to the coils can be selected to
control spatial distribution of the plasma within the hollow
form.
[0028] A main drawback of all apparatuses and methods for
application of coatings onto the inner surfaces of containers known
to the inventor is non-optimal direction of the magnetic field
generated by the antenna coils. RF power applied to these coils
provides RF current that generates an axial magnetic field.
Therefore, plasma density in such systems is distributed so that
maximum plasma density is concentrated in the vicinity of the axis
but minimum plasma density is close to the inner wall of the
container, when the antenna is used for plasma-enhanced chemical
vapor deposition of a barrier layer onto the inner walls of the
aforementioned container. Coating of the walls in such a system has
a low throughput rate. In other words, the existing antennas of
apparatus for treating inner surfaces of containers have a geometry
that does not produce plasma fields that match the inner profiles
of containers.
OBJECTS AND SUMMARY OF THE INVENTION
[0029] It is an object of the present invention to provide an
antenna assembly with an RF antenna that generates a transverse
magnetic field perpendicular to the longitudinal axis of the
antenna for generation of inductively coupled plasma. It is another
object to provide an antenna assembly of the aforementioned type,
which is suitable for efficient application of thin
fluid-impermeable barrier coatings onto inner surfaces of
containers, such as bottles, especially by generating ICP discharge
plasma in a PECVD process. It is a further object to provide the
aforementioned antenna assembly wherein the antenna has a
three-dimensional shape tailored for specific profile of the inner
walls of hollow containers such as bottles and capable of
sustaining plasma inside the containers with plasma density uniform
and increased in the vicinity of the inner walls of the container.
It is a further object to provide the aforementioned antenna
assembly, which is suitable for application of one or more layers
of silicone dioxide coatings onto the inner surfaces of hollow
containers at relatively low temperatures, with high-speed rate of
deposition and with possibility of controlling the coating material
deposition process. It is a further object to provide the
aforementioned antenna assembly suitable for use in high throughput
systems under mass production conditions. It is a further object to
provide the aforementioned antenna assembly suitable for
incorporation into a high-speed automated production line for
forming an array of collectively controlled antennas for
simultaneous coating inner surfaces in a plurality of containers
and for subsequent filling of the containers with beverages without
the need for sterilization, which is eliminated from the production
process due to the use of the antenna of the invention. It is a
further object to provide an array of the RF antennas of the
aforementioned type suitable for treatment of a plurality of
containers without discontinuing the supply of RF power to the
coating stations but rather redistributing the power from the
coating stations to dummy loads during noncoating periods in the
working cycle.
[0030] An antenna assembly of the invention consists of an antenna
holder that supports the RF antenna in a position coaxial with the
longitudinal axis of the container to be treated and that also
supports a gas-supply tube and is provided with gas exhaust
openings. Also, the antenna holder is used as a cap for sealing the
mouth of the hollow container. More specifically, the antenna
assembly of the invention contains an RF antenna for generation of
inductively coupled plasma and capable of creating a magnetic
fields transversal to the axis direction of the antenna, i.e., to
the longitudinal axis of the container into which the antenna is
inserted for deposition of the barrier coating. The antenna is
comprised of at least two windings, which are connected in series
or parallel, azimuthally distributed relative to the axis of the
mouth of the hollow container, when the antenna is used in an
apparatus for application of fluid-impermeable coatings onto the
inner surface of the containers. The antenna is energized by the RF
current that generates magnetic fields normal to the lateral
surfaces of the aforementioned hollow containers. In certain
aspects, each winding of the transversal antenna is a solenoid
faced to the wall of the hollow container, having a rectangular,
elliptical, oval, or other configuration obtained by wrapping onto
a transversal mandrel with a radius of curvature equal to or less
than the radius of curvature of the aforementioned mouth through
which such transversal RF antenna is immersed inside the hollow
container.
[0031] Distance d between neighboring turns of the spiral coil is
related with a breakdown voltage V.sub.B as:
V.sub.B=A pd/ln(pd)+B,
[0032] where p is pressure in the volume into which the antenna is
immersed, pd is the Paschen minimum, and A and B are constants,
depending on geometry of the antenna.
[0033] In certain embodiments, input of the first winding and
output of the last winding are connected to the terminals of the
matching network that is connected to the RF generator.
[0034] Further, in certain specific embodiments wherein the antenna
assembly of the invention is used in the apparatus for application
of a barrier layer onto the inner surface of a container, the first
winding of the transversal antenna is provided with an igniting
solenoid positioned near the bottle neck, and input of the first
winding is connected to the matching network through the igniting
solenoid. Also, in certain embodiments, aforementioned igniting
solenoid, together with an argon supply tube enveloped by such a
solenoid, constitutes a generator of charged particles triggering
the ICP discharge inside the hollow container.
[0035] The central gas tube enveloped by the transversal antenna
and used for delivering the process gas into the vacuum chamber is
provided with holes that may have non-uniform diameters and/or
distributed nonuniformly on the lateral surface for providing
instant uniformity of a precursor-gas flow in the container. This
distribution depends on the curvilinear profile of the inner
surface of the hollow container and geometry of a three-dimensional
winding that matches the aforementioned profile.
[0036] Distribution of the injected gas, controlled by nonuniform
distribution of these holes, adjusts the plasma density that, in
turn, provides substantially uniform and continuous deposition of
the barrier layer from the generated plasma resulting from the
plasma chemical reaction.
[0037] In the apparatus for application of barrier coatings onto
the inner surfaces of containers by using the antenna assembly of
the invention for generation of inductively coupled plasma, each
coating station comprises a transversal RF antenna mounted on the
antenna holder and a winding that matches the profile of the
aforementioned curvilinear inner surfaces. The dimensions of this
winding allow its penetration through the container's mouth into
the container during haft of this container on the antenna. The
antenna holder seals the inner container volume after contact with
the lip of the neck of the container. A gas supply tube supplies
this volume by the process gas that is a mixture having a first gas
component provided by evaporation of organosilicon liquid. A second
gas component is oxygen provided from an oxygen container.
Altogether, these assembled components constitute an individual
PECVD chamber where the plasma chemical reaction occurs. Suitable
organosilicon liquids include siloxanes such as
hexamethyldisiloxane (HMDSO), 1, 1, 3, 3-tetramethyldisiloxane
(TMDSO), and octamethylcyclotetrasiloxane; alkoxysilanes such as
amyltriethoxysilane, ethyltriethoxysilane, isobutyltriethoxysilane,
and tetramethoxysilane; silazanes such as hexamethyldisilazane; and
fluorine-containing silanes such as trimethyfluorosilane.
[0038] The plasma-enhanced chemical vacuum deposition of the
barrier layer from the process gas is carried out with plasma,
which is not interrupted but rather constantly sustained
alternatively either in a high-density mode capable of causing a
plasma chemical reaction between the SiO.sub.2-generating gases or
in a low-density glow discharge mode where RF power is not high
enough to provide a plasma chemical reaction for barrier-layer
deposition but enough to generate the charged particles in the
container in order to maintain the system in readiness in order to
restore quickly the deposition process. The dummy load provides
transformation from one mode to another. The drawback of abrupt
discontinuing of plasma in a conventional mode of deposition of a
barrier layer is that a new breakdown is needed to start the
discharge again. However, in accordance with the preferred practice
of this invention, the ICP discharge generated by the transversal
antenna is depleted by reducing RF power, which is not completely
interrupted. Continuous presence of plasma helps to save charged
particles, and a new breakdown is not needed. In accordance with
the invented method, discharge is depleted by intercepting the
power with the RF power dummy loads that are comprised of a sealed
vessel, or vessels, filled with an inert gas, e.g., argon. The
discharge in the dummy load is triggered by the igniter, which is
connected to the high-voltage pulse generator. The period of
sustaining plasma in the dummy loads is used for cooling the
plastic containers from overheating with an elevated
temperature.
[0039] More specifically, the discharge in each dummy load is
coupled with the transversal electromagnetic field of the antenna,
absorbing a valuable part of the RF power. The igniter comprises an
electrode immersed in the argon volume and generates a spark for
breakdown of the dummy load's gas volume.
[0040] In accordance with the invention, the high-voltage pulse
generator serves as an alternator of the PECVD process from the
deposition mode to the cooling mode. The high-voltage pulse
generator forms a pulse interval in order to cool the inner walls
of the containers with process gas.
[0041] The transversal antenna comprises a plurality of coils
distributed regularly relative to the axis of a hollow container,
wound in the orthogonal direction, and configured in accordance
with the inner profile of the hollow container or the mouth of the
container, such as a bottle. The antenna generates a plurality of
electromagnetic fields directed to the inner walls of the hollow
container and converted into a plurality of the electrical fields
that, in own turn, are converted into relatively uniform plasma
having increased density near the walls of the containers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a three-dimensional view of the ICP transversal
antenna of the invention.
[0043] FIG. 2 is a top view of the ICP transversal antenna of FIG.
1, the antenna being composed of spiral coil parts arranged in flat
planes.
[0044] FIG. 3 is a view similar to FIG. 2 but for the transversal
antenna composed of spiral coil parts which in a top view have
curvilinear or saddle-like profiles.
[0045] FIG. 4 shows a modification of the transversal antenna of
the present invention, the antenna having an elongated shape
suitable for insertion into a bottle with a narrow neck and a
saddle-shaped profile in a top view for matching to the shape of
the bottle inner wall.
[0046] FIG. 5 is a three-dimensional view of the transversal
antenna of the invention that consists of four curvilinear solenoid
parts inserted into a container of a tapered shape, such as a wine
goblet.
[0047] FIG. 6a is a cross-section of the vacuum chamber with the
array of individual PECVD chambers that comprise coating stations
formed in the containers.
[0048] FIG. 6b is a cross-section of the vacuum chamber with a
clamping device.
[0049] FIG. 6c is a total view of the barrier-coating production
line.
[0050] FIG. 6d is a schematic view of the process of coupling the
antenna and dummy loads.
[0051] FIG. 6e is a schematic view illustrating the process of
aligning the container.
[0052] FIG. 6f is an exploded view of a clamping device with a
neck-capturing mechanism.
[0053] FIG. 6g is a view illustrating the process of capturing the
necks of containers by the neck-capturing mechanism.
[0054] FIG. 6h is a view of the top sliding strip of the
neck-capturing mechanism with cam and spring.
[0055] FIG. 6k is a view of the bottom sliding strip of the
neck-capturing mechanism with a cam and a spring.
[0056] FIG. 6l is a view of the platform of the neck-capturing
mechanism.
[0057] FIG. 6m is a view illustrating the sequence of operations
for the formation of a barrier coating.
[0058] FIG. 6p is a view illustrating the alignment process carried
out by means of an array of expandable fingers.
[0059] FIG. 6q is a view illustrating immersion of the array of the
expandable fingers into the necks of the containers.
[0060] FIG. 6r is a view illustrating the sealed chamber with the
containers halted on the coating stations.
[0061] FIG. 7 is a three-dimensional view illustrating a
three-dimensional saddle-shaped transversal RF antenna of the
invention inside a container, e.g., a bottle into which the
precursor gas is supplied through openings in the gas distribution
tube.
[0062] FIG. 8 is a view similar to FIG. 7, wherein the transversal
antenna assembly is provided with a solenoid that together with an
argon delivery tube constitutes a discharge plasma ignition trigger
for initial ignition of plasma in the container, e.g., a
bottle.
DETAILED DESCRIPTION OF THE INVENTION
[0063] The inventor herein has found that in contradiction with the
generally accepted erroneous viewpoint that the magnetic field of a
solenoid has maximum strength on the longitudinal axis thereof, in
reality the strength of the magnetic field on the longitudinal axis
of the solenoid is minimal, while the main part of the lines of the
magnetic flux is concentrated near the inner area solenoid winding.
In order to check this statement it is sufficient to consider a
magnetic field of a circular turn through which a current flows and
then to summarize (integrate) the result over the solenoid length.
Contrary to this, an electric field generated in plasma surrounded
by a solenoid has its maximum in the center of the solenoid and has
its minimum in the vicinity of the solenoid's winding. Density of
plasma has the same distribution pattern. Therefore, in the past
the design with the maximum axial brightness was chosen as a
plasma-based source of light. However, such a design where density
is maximal in the center of the container and minimal near the
inner walls thereof is disadvantageous for application of barrier
layers onto the inner surfaces of containers. In the latter case,
the speed of deposition of the coating material will also be
minimal.
[0064] The present invention is based on the above finding made by
the inventor.
[0065] The transversal antenna assembly of the invention is
intended for generation of inductively coupled plasma (ICP) in
sealed and evacuated containers. The term "transversal" includes an
antenna with saddle-like coils wherein all turns of the antenna
winding are formed by wrapping a wire around a cylindrical mandrel
that has a diameter less than the diameter of the container's
mouth, if the mouth is round. For a cylindrical mandrel, the turns
of each saddle-like coil may have an angular, elliptical, oval,
rectangular, tapered, or nontapered configuration, depending on the
configuration of the hollow container. The turns inherit the same
bending radii as the mandrel. Taking into account the fact that the
antenna is immersed into the gaseous volume, all turns are
separated from each other to prevent high-voltage breakdown between
neighboring turns.
[0066] One may express the relation between breakdown voltage
V.sub.B and distance d between turns as:
V.sub.B=Apd/ln(pd)+B, (1)
[0067] where p is pressure in the volume where the antenna is
immersed, d is critical distance, pd is the Paschen minimum, and A
and B are constants, depending on geometry of the antenna.
[0068] Each next turn has an increased width and height increased,
as compared with the central turn. The determination "transversal"
includes an antenna with solenoid-like coils, wherein the turns are
formed by wrapping the wire around several azimuthally arranged
mandrels, which are joined to the central axial mandrel of the
constitutive fixture. In this case, the turns have the same
geometry but are separated in the radial (relative to the axis of
the mouth) direction with clearance that is large enough to prevent
high-voltage breakdown between neighboring turns. The determination
"transversal" also includes an antenna with coils wherein the turns
are wrapped as a solenoid with a radially increased size. Each turn
can be larger in the radial direction to the wall of the hollow
container in order to fill out the total space of the hollow
container by the wire, especially of the container without a neck,
in order to develop a plasma column in the vicinity of the inner
surface of the container inside the narrow space between the inner
surface of the container and the front turns of the solenoids.
[0069] Also, the radius of curvature of an outward turn of each
solenoid is increased as compared with the inward turn. In this
case, high uniformity of coating and high rate of deposition can be
provided with relatively low RF power.
[0070] The geometry of each turn of each spiral coil can be
different. In a two-dimensional projection, it can be rectangular,
tapered rectangular, elliptical, or oval, with the plane of
symmetry coincident with the axis of the mouth of the hollow
container and normal to the inner surface of the container. In the
axial projection, the spiral coils are distributed angularly and
uniformly with increments of 180/n, where n=1,2,3 . . . m.
[0071] The transversal antenna is comprised of multiturn coils
connected in series or in parallel. The coils can be spiral coils
or solenoids azimuthally distributed relative to the axis of the
mouth of the hollow container through which the antenna is immersed
into the hollow container.
[0072] Each turn is made from a copper wire or a copper tube for
passing a cooling medium, and an outward turn of each coil is bent
with a radius of curvature equal to or less than the radius of
curvature of the mouth of the hollow container. For a container
without a mouth or pipe, each outward turn of each coil is bent
with a radius of curvature less than the radius of curvature of the
inner surface of such container or pipe. It is understood that for
an open container, such as a cap, wine goblet, etc., the outward
turn approaches the inner surface with a distance sufficient enough
to provide a high rate of deposition and high uniformity of the
barrier coating but with a clearance that prevents melting of
plastic during deposition.
[0073] Although the transversal antenna immersed in the hollow
container generates the same ICP discharges as the axial solenoid
immersed in the RF light bulb, distribution of plasma density in
these discharges is different because of the different direction of
magnetic flux induced by such antennas. The magnetic flux of the
axial solenoid antenna is directed along the longitudinal axis of
the bulb and transforms the high electric current into plasma
oriented in the direction of this longitudinal axis. Accelerated
electrons have higher ionization efficiency. They create higher
plasma density along the axis and in the vicinity of the bottom of
the bulb in order to produce high axial brightness of the plasma.
The transversal antenna of the invention directs the
electromagnetic field toward the walls of the hollow container.
Because of this direction, electromagnetic fields of the several
angularly distributed coils are transformed into plasma by high
current in the vicinity of the inner surface of the hollow
container. The higher the ionization efficiency and the greater the
electron density near the inner surface of such a container, the
higher is the rate of the deposition of SiO.sub.2 from plasma, and
the higher is the intensity of the chemical reaction between
organosilane and oxygen.
[0074] Simultaneously with deposition, the plasma heats the
interior surface of the container, and this, to some limit,
increases density of the deposited coating and enhances barrier
properties thereof. The duration of the deposition must be very
short; otherwise, the plastic can be softened, even molten, and the
hollow container can collapse. The outward turn approaches the
inner surface within a reasonable limit.
[0075] The azimuthally distributed coils provide uniformity of such
deposition. The transversal antenna comprises an even number of
sets of windings that can be connected in series or in parallel. In
any case, the direction of the electromagnetic fields of the
opposite coils is supposed to be the same; otherwise, the total
electromagnetic field would be weakened, plasma density would be
reduced, coating would be nonuniform, and impedance of the antenna
and reflected RF power would be increased. The angular increment of
the azimuthally distributed coils is supposed to equal 180/n, where
n=1,2,3, . . . m. It is understandable that the higher the n, the
more uniform the thickness of the deposited thin film.
[0076] A three-dimensional view of a transversal RF antenna 20 of
the invention is shown in FIG. 1. Since the ICP transversal
inductive antenna 20 has a three-dimensional configuration,
positions of some parts of the antenna 20 will be considered with
reference to an orthogonal XYZ coordinate system, as shown
schematically in FIG. 1.
[0077] In the embodiment shown in FIG. 1, the saddle-like version
of the transversal antenna has a winding 22 that consists of two
parts, i.e., an ICP transversal antenna winding part 22a arranged
as a separate coil substantially in a first XZ plane 24a and an ICP
transversal antenna winding part 22b, which is arranged as another
separate coil in a second XZ plane 24b. Input of the first coil and
output of the second coil arrangements are connected to an RF power
supply (not shown) that provides RF current flowing in the same
direction in both coils. Such an arrangement allows the total
magnetic flux, produced by both coils, to be increased. This total
magnetic flux, which is shown by arrow M in FIG. 1, is transversal
relative to the axis of the mouth of the container and normal to
the walls of this container (shown and described below, e.g., a
bottle 422 shown in FIG. 7). The total magnetic flux transforms the
electrical field in plasma that is close to the walls. It is
understood that in the embodiment shown in FIG. 1, the planes 24a
and 24b and, hence, the winding parts 22a and 22b, are shown
schematically as flat. It is understood that they are being wrapped
on a lateral surface of the mandrel and, in reality, inherit a
radius of curvature equal to or less than the radius of curvature
of the mouth of the container. It is also understood that the turns
need not be arranged on a flat plane along curvilinear profiles,
such as cylindrical or taped cylindrical profiles, which, depending
on the radius of curvature can be equal to or less than the radius
of the container's mouth. It is understood that the turns can be
circular, rectangular, rectangular tapered, elliptical, oval, or of
another shape. In this case, each next turn will have an inner area
larger than the previous area, and the distance between the
neighboring turns must exceed one critical from the viewpoint of
high potential breakdown between the neighboring turns.
[0078] The first antenna winding part 22a may have two or more bent
turns that may have different configurations and dimensions
selected in compliance with the specific object and object profile
to be treated. For example, configuration of the turns may be
rectangular, rectangular tapered, circular, elliptical, or oval. In
the specific embodiments shown in FIG. 1 for the purposes of
example only, the antenna winding part 22a has a spiral shape that
consists of a small oval-shaped turn 22a1 and a large oval-shaped
turn 22a2. It is understood that the oval shape of the turns is
shown only as an example and that the turns 22a1 and 22a2 may have
a round, rectangular, tapered, or nontapered configuration.
[0079] An input terminal 26 of the large oval-shaped turn 22a2 of
the first antenna winding part 22a is connected through a matching
network (not shown in FIG. 1) to the first terminal of an RF power
source (not shown in FIG. 1), while an output terminal 28 of the
first antenna winding part 22a is connected to an input terminal 30
of the second winding part 22b. An output 32 of the second winding
part 22b is connected through the matching network to the second
terminal of an RF power source (not shown in FIG. 1). It is
understood that the entire circuit from the input terminal 26 of
the first winding part 22a to the output terminal 32 of the second
winding part 22b is continuous and has a series connection. Arrows
show direction of the current that provides the electromagnetic
field in each winding part with the same direction.
[0080] FIG. 2 shows a top view of the transversal RF antenna 20. It
can be seen from FIG. 2 that in the embodiment of the antenna 20
shown in FIG. 2, the first and second antenna winding parts 22a and
22b are located in mirror positions in parallel planes and
therefore have flat configurations. However, the antenna winding
parts 22a and 22b may also have curvilinear configurations
inherited from the curvilinear configuration of the mandrel (not
shown in FIG. 2) onto which they are wrapped according to the
radius of curvature of the mouth of the container or according to
the radius of curvature of the curvilinear inner surface of the
open container. This is shown in FIG. 3, which is a top view of the
antenna similar to one shown in FIG. 2 and is designated by
reference numeral 20'. Parts of the antennas shown in FIG. 3,
similar to those of the antenna 20 in FIGS. 1 and 2, are designated
by the same reference numerals but with the addition of a prime.
For example, in the antenna 20', the input terminal of the large
oval-shaped turn of the first antenna winding part 22a' is
designated by reference numeral 26', and so forth.
[0081] FIG. 4 shows a modification of antenna 120 of the present
invention, which has an elongated shape suitable for insertion into
a bottle with a narrow neck and a saddle-shaped profile in a top
view to match the shape of the inner wall of a bottle. Parts of the
antennas shown in FIG. 4, which are similar to those of the antenna
20 in FIG. 1, are designated by the same reference numerals but
with the addition of 100. For example, in the antenna 120, the
input terminal of the large oval-shaped turn of the first antenna
winding part 122a is designated by reference numeral 126, etc. In
antenna 120 of FIG. 4, reference numeral 140 designates a solenoid
joined to the antenna coils that can envelop a second gas tube
connected to the reservoir with argon and constitute with such a
gas tube a trigger for ignition of plasma discharge. The trigger
coil 140 is connected in series with the output 132 of the winding
part 122b of the antenna 120. Topology of windings in the antenna
120 is similar to the topology of the first and second winding
parts of the antenna 20, but the shape of the winding parts is
elongated and matches the shape of the inner walls of a bottle for
uniformity of the coating to be applied onto these inner walls. The
antenna of the invention is not limited to cylindrical
configurations and may be outlined for treating containers having
inner walls of conical, semispherical, or other curvilinear shapes,
e.g., shapes of varying curvatures. The antenna 220 of the
invention modified for treating, e.g., conical surfaces, is shown
in FIG. 5, which is a three-dimensional view of the antenna 220
inserted into a wine glass 221. It is understood that the antenna
220 and the wine glass are located in a closed-volume vacuum
chamber (not shown). The antenna 220 is similar to one shown in
FIGS. 1 and 2 and consists of four antenna parts, or four coils
222a, 222b, 222c, and 222d. Each coil consists of two or more
turns, an outward turn and an inward turn, with the radius of
curvature decreasing toward the center. The difference in radius is
greater than the threshold for breakdown between neighboring turns.
In order to conform to the tapered shape of the glass wall 221a,
the antenna parts 222a, 222b, 222c, and 222d taper in the downward
direction (i.e., they are inclined relative to vertical axis Z, as
shown in FIG. 1). The structure of each antenna part is the same as
in the antenna of FIG. 1.
[0082] In addition to antenna design, two aspects provide thermal
preserving of the plastic material during coating and arrangement
in the stationary position of the units. The function of these
units is to generate electromagnetic energy and to supply process
gas to the coating station. The periodical disconnect of these
units from RF power, process gas, water supplies, and pumping
communication would, therefore, hugely increase the burden on the
apparatus for PECVD. To prevent damage to the plastic material,
especially biodegradable plastic, by excessively high plasma
temperature, thermal flux generation periods are alternated with
periods of cooling and removing byproducts of the plasma chemical
reaction. This happens while providing activating energy needed to
decompose the process gas for generation of silicon dioxide that is
deposited onto the inner walls of the plastic containers. In the
context of the process according to the invention, it is
appropriate to use pulsed plasma to allow material-preserving
coating of the temperature-sensitive biodegradable plastic
substrate. A pulsed plasma ratio, also known as a duty ratio, is
defined as the ratio of pulse duration to the pulse space interval.
During pulse duration, on one hand, a great amount of RF power is
supposed to be introduced to ignite and to sustain plasma
discharge. On the other hand, thermal shock of hot plasma generated
by such RF pulse, particularly for biodegradable plastic, can be
drastically reduced during the following pulse space interval when
byproducts are removed. Cold-process gases and auxiliary gases,
such as argon, cool the inner surfaces of the plastic
containers.
[0083] Although the apparatus for application of a barrier coating
onto the inner surfaces of containers is beyond the scope of the
present invention, it would be advantageous to describe such an
apparatus since the antenna assembly of the present invention
constitutes a main part of such an apparatus and description of the
apparatus will contribute to better understanding of the present
invention.
[0084] As shown in simplified form in FIGS. 6a, 6b, 6c, 6d, the
apparatus of invention provides simultaneous application of barrier
coatings by PECVD process for a plurality of containers. The
apparatus 301, shown in FIG. 6a, provides simultaneous barrier
coating for an array 302 of containers 303a through 303n. The
apparatus 301 comprises a vacuum chamber 304 that consists of a
coating panel 305 with an array 306 of the coating stations 307a
through 307n (FIG. 6b). The panel 305 is maintained on the back of
the chamber 304. The coating stations 307a through 307n are
immersed into the chamber 304. The chamber 304 also comprises a
front flange 308 with a door 309 (FIG. 6a) maintained on the shafts
310a and 310b, which are driven by the linear actuators 311a and
311b.
[0085] The door 309 serves as a periodical opening of the chamber
304 when it is necessary to introduce an array 302 of the
containers 303a through 303n into the chamber 304 for deposition of
the barrier layer or for removal of the containers after the
deposition process is completed. During deposition, all containers
303a through 303n are locked in a movable clamping device 312 (FIG.
6b). The clamping device 312 secures positions of the containers
303a through 303n (FIG. 6a) with the same distance between
containers in the array 302 as the distance between coating
stations 307a through 307n. The clamping device 312 also provides
capturing of the containers 303a through 303n from the flat part
313 of the belt 314 of the conveyer 315 and delivers the deposited
containers 303a through 303n to nests 316 in the belt 314 of the
same conveyer 315 (FIG. 6c). More specifically, the conveyor belt
314 has a flat portion 313 on which the containers are aligned and
a nested portion 316 with nests that are strictly aligned with the
containers in their aligned state and into which the clamping
device places the aligned containers.
[0086] In order not to overload the drawings and specification,
some multiple components, e.g., containers 303a through 303n will
be designated by a single reference numeral, e.g. instead of
containers 303a through 303n, each container will be designated by
a single reference numeral 303, although it is understood that the
number of containers is "n" and corresponds to the number "n" of
filling stations 307a through 307n, etc. Nests 316 serve as
cartridges to secure the interval between the containers 303, which
is also equal to the interval between nozzles of a filling device
(not shown) whereto the containers are delivered. The clamping
device 312 provides capturing and holding of the containers 303
using the neck-locking mechanism 317.
[0087] Movement of the neck-locking mechanism 317 of the clamping
device 312 is performed by using arms 318a and 318b driven by
linear actuators 319a and 319b and used as carriers of the array
302 of the containers 303 (see FIG. 6b). Evacuation of air from
volume 320 of the vacuum chamber for deposition of the barrier
layer on the inner walls of the containers 303 is provided by a
pump 321 through a vacuum valve 322. Control of pressure inside the
chamber 304 is provided with a Baratron 323 (FIGS. 6a and 6c).
[0088] The chamber 304 also includes two quartz windows 324a and
324b situated on both sides of the chamber 304 (FIG. 6d). Each
coating station 307 comprises a transversal antenna 325, antenna
holder 326, and a process gas supply tube 327.
[0089] Each container is periodically clamped to the respective
antenna holder 326. The coating stations 307 and respective
enveloping containers 303a through 303n sealed by the surface of
the antenna holders 326 constitute individual PECVD chambers 328
that serve to deposit barrier layers 329 of the silicon dioxide
onto the inner surfaces of the containers 303a through 303n.
[0090] Each transversal antenna 325 of the coating station 307 has
a high RF current connection with a matching device 330 and an RF
generator 331 that applies RF current to the antenna 325 through a
terminal 332 (FIG. 6d). Another terminal 333 of the antenna 325 is
grounded. The gas feeding tube 327 with nonuniformly distributed
holes 334 on the lateral surface serves to deliver a process gas to
the volume 335 of the container 303 (FIG. 6a). After being filled
with process gas, the volume 335 is sealed with the lip 337 of the
neck 338 of the container 303 against the surface 336 of the
antenna holder 326.
[0091] Each gas supply tube 327 of the coating station 307 is
connected to the gas manifold 341, which uniformly distributes the
process gas between volumes 335 of the individual PECVD chambers
328. Through the process gas flow controller 343, the manifold 341
is connected with a mixer 344, an organosilane vaporizer 345, and a
tank 346 that contains organosilane (TEOS), as shown in FIG. 6a.
Oxygen is delivered to the mixer 344 from the oxygen tank 347
through the oxygen flow controller 348. This mixture is used for
the plasma chemical reaction that is caused in each container 303
under the effect of plasma discharge 349 generated in each
container by the transversal antenna 325. (FIG. 6d). Silicone
dioxide, which is a product of the plasma chemical reaction, is
deposited onto the inner walls 329 of each container 303 by forming
a fluid-impermeable barrier layer 329 (FIG. 6d). Pumping of the
volume 335, separated from the volume of the chamber 304 inside the
containers 303, and removing byproducts of the plasma chemical
reaction, such as carbon dioxide and water, from this volume 335
are carried out by a pump 350 (FIG. 6a). Vacuum communication is
provided through a sealed inner volume 351 of a hollow panel 305
that serves as a vacuum manifold 352 pierced by the antenna holders
326 and through the oblique holes 353 formed in the antenna holder
326.
[0092] In order to preserve the plastic material of the containers
from a destructive heat flux irradiated by plasma, periodical
cooling of the inner wall of the containers 303 is to be provided.
The discharge shunting system 354 provides this goal comprises two
dummy loads 355a and 355b, which are similar to the commercial
light panels positioned outside the chamber 304 behind the windows
324a and 324b (FIG. 6d).
[0093] Each dummy load consists of a glass vessel 356 filled with
argon and provided with an electrode 357 connected to a pulse
generator 358. This generator 358 generates sequence of
high-voltage pulses 359 with intervals between pulses 360 and
serves as a trigger of plasma discharge 361 in the vessel 356.
[0094] The apparatus is provided with an array 306 of coating
stations 307 spaced at intervals equal to intervals between the
nozzles of the filling system into which the containers are fed on
completion of treatment in the apparatus of the invention (FIG.
6b).
[0095] Such equalization allows for integration of the
barrier-coating system of the invention into a commercial
production line designed for filling the containers with beverages
or other contents, with subsequent capping and labeling.
[0096] The conveyor 315 transports randomly positioned containers
303 from a supply hopper of the conventional container supplier
(not shown). This conventional conveyor removes randomly oriented
containers from the supply bin and then orients and roughly
sequences the containers in a bottom-up, top-down orientation. Then
the conveyor transports the containers to a turning plate and to a
drop-chute, which turns the bottom-up, top-down containers to the
top-up, bottom-down position. However, the aforementioned
conventional system does not provide precise positioning and
sequencing of these containers with equal distances between each
other, which is needed for processing the containers in the area of
the coating. In order to satisfy this requirement, the containers
must be aligned and organized into an array, with intervals equal
to those in the array of the RF antenna in the coating stations.
The filling process also requires organization of the containers
into an array, with distances between the containers that match
those in the array of the coating stations. If in the array 302 the
containers 303 are distributed with the same intervals as the
coating stations and the filling nozzles of the filling station,
sterilization of the containers can be omitted immediately after
barrier coating and before filling the containers with a
beverage.
[0097] Multinozzle filling of the containers with beverage is a
process that must be carried out with high throughput and with
short intervals between filling cycles, wherein movement of the
conveyor is interrupted during filling. This means that the time
allowed for the barrier-coating cycle will also be very short. On
the other hand, cycle time of the coating system can be neither
longer nor shorter than the cycle time of the filling station, and
the entire production line should operate within the same operation
cycle. In order to allow sufficient time for the coating cycle, it
is necessary to accelerate portions of the process cycle such as:
positioning of containers 303 on the conveyer 315 in the array 302,
delivery of aligned containers 303 to the process chamber 304,
sealing and evacuation of the vacuum chamber 304 with the coating
stations 307, coating with a high deposition rate, and return of
the coated containers to the conveyer 315.
[0098] It is understood that each container must be precisely
aligned with the longitudinal axis of the transversal antenna 325
(FIG. 6a).
[0099] Thus, aligning is an important part of the vacuum
barrier-coating process. An aligning system is integrated into an
entire coating apparatus, wherein the clamping device 312 (FIG. 6c)
traveling from the conveyer 315 to the coating chamber 304 and back
belongs to both the conveyor and coating chamber. This clamping
device 312 holds the containers in the vacuum chamber 304 during
the barrier coating and catches the necks of the containers 303 in
the aligned position, which is provided by the previously acting
alignment mechanism.
[0100] The method of alignment preceding the process of barrier
coating is working in a manner such that the containers 303
randomly positioned on the flat part 313 of the belt 314 of the
conveyor 315 are automatically organized in an array with uniform
distribution. The containers of the array are locked in order to
secure the position. Means that provide such an operation
include:
[0101] 1. An aligning mechanism in the form of pipettes made from
an inflatable material, such as rubber or plastic, that may have an
inflated state and a deflated state. In the deflated state, the
expandable members have diameters smaller than the openings of the
containers, and in the inflatable state, the expendable members
have diameters greater than the diameters of the containers so that
when the pipettes are inflated, they are deformed into the form of
balls, which are pressed against the inner walls of the container
necks, and act as universal joints that provide self-alignment and
shift the containers from unaligned positions to aligned
positions.
[0102] 2. A clamping device with an array of holes narrowing in
size for capturing the necks of the containers.
[0103] Combined variability of the clamping and aligning devices
allows for precise positioning of the containers and simultaneously
for securing thereof in the aligned positions during deposition of
the barrier layers.
[0104] This alignment method described above precedes the clamping
step and provides positioning of the containers sufficient enough
for catching the necks of the containers with the narrowing holes
of the clamping device.
[0105] The container locking system 361 incorporates the clamping
device 312 illustrated in FIG. 6e. The locking system 361 (FIG. 6c)
consists of a two-directional movement system 362 provided with
horizontal (363) and vertical (364) actuators for movement of the
slide aligning station 365 in two directions. Movement of the slide
366 and, hence, of the aligning station 365 supported by this slide
366 in the vertical direction, is carried out by the vertical
linear actuator 364 along vertical guides 367a and 367b supported
by a base 368 (FIG. 3c). Movement of the aligning station 365 in
the horizontal direction along horizontal guides 369a and 369b is
performed by a horizontal linear actuator 363. The horizontal
linear actuator 363, as well as the horizontal guides 369a and
369b, is connected to the slide 366. The aligning station 365 is
comprised of a manifold 370 joined to an air compressor 371 that
supplies the manifold with compressed air and a vent 372 for
draining air from the manifold 370. The aligning station 365
includes an array 373 of inflatable rubber fingers 374 on the end
of the tubes 375 which are connected to the manifold 370. These
tubes 375 serve as carriers of the fingers 374. They provide air
communication between the manifold 370 and the fingers 374 in order
to supply each finger 374 with pressurized air or to remove air
from the fingers. All fingers 374 are organized into the array 373
with the same intervals as the intervals between the coating
stations 306. Each finger 374 has a mushroom-like structure and
consists of a pipette 376 hafted on the tube 375 and crowned by a
rubber hat 377 that clamps the pipette 376 on the end of tube 374
(FIG. 6e). In fact, the pipettes 376 are made from an inflatable
material, such as rubber or plastic, and may have an inflated state
shown in FIG. 6e or a deflated state. In the deflated state, the
expandable members have diameters smaller than the openings of the
containers. In the inflated state, the expendable members have
diameters greater than the diameters of the containers. Therefore,
when the pipettes 376 are inflated, they are deformed into the form
of balls 379, which are pressed against the inner walls of the
container necks 338, and act as universal joints that provide
self-alignment and shift the containers from unaligned positions to
aligned positions.
[0106] Being carried by the aligning station 365, the array 373 of
the fingers 374 can move horizontally in order to be positioned
against the containers 303 distributed randomly on the belt 314 of
the conveyer 315 (FIG. 6e). After alignment with the axes of the
coating stations 307, the fingers 374 can be moved down by the
vertical linear actuator 364 for immersing into the throats 378 of
the necks 338 of the containers 303 and then up in order to contact
the hat 377 with the lip 337 of the neck 338 (FIG. 6c and FIG.
6e).
[0107] Although the container 303 may be misaligned with the axis
of the coating station, each rubber pipette 376 still can occupy a
small part of volume of the throat 378 (FIGS. 6c and 6e). After
being blown out by pressurized air, each pipette 376 can expanded
in the form of a ball 379 in order to fill the total volume of the
throat 378 and to develop pressure against the inner walls of the
container neck 338, thus holding the container 303. Since the
inflated balls 379 act as universal joints that hold the containers
303, they move the containers into positions aligned with the axes
of the respective fingers 374. The rubber hats 377 that press the
lips 337 of the necks 338 against the belt 314 limit movement of
the containers 303 (other than horizontal movement) for alignment
with the axes of the coating stations.
[0108] A neck-locking mechanism 317 of the clamping device 312
shown in FIG. 6f secures alignment of the containers 303 after
removal of the aligning fingers 374 from the necks 338 of the
containers. Capturing and locking necks 338 are provided with an
array 380 of orifices 381 having variable apertures (FIG. 6g). The
number of these apertures is equal to the number of the containers
303 in the array 302. These apertures can be large enough to
capture the necks 338 of the containers 303 randomly distributed on
the belt 313, 314 of the conveyer 315, 314. After alignment, the
apertures of these orifices 381 can be narrowed to the size of the
necks 338 in order to lock these necks by the clamping device 312
during delivery of the containers 303 to the vacuum chamber 304 for
coating. Such a reduced aperture holds the containers 303 during
deposition of the barrier layers as well as during return of the
treated containers to the nested belt 316 of the conveyer 314. The
same apertures of the orifices 381 can be enlarged in order to
unlock the necks 338 after completion of handling the containers
303 in the nests 316 of the belt 314. The array 380 of the variable
orifices 381 is formed by superposing holes 382 (FIG. 6f) of two
top and bottom sliding strips 383a (FIG. 6h) and 383b (FIG. 6k),
respectively, which move in opposite directions and are sandwiched
between the top and bottom platforms 384a and 384b (FIG. 6l). Each
platform has an array of large holes 385 (FIG. 6l). However, the
geometry of holes 382 and 385 is different. While the holes 385 are
round, the holes 382 are oval, with large and small radiuses of
curvature. Superposition of the curvilinear parts 386 of the holes
382 with a large curvature form large orifices for embracing the
necks 338 of the containers 303 with rough distributions on the
belt 314 (FIG. 6g). Superposition of the narrowing edges 387 of the
oval holes 382 forms small orifices capable of squeezing the necks
338 of the containers 303 in order to keep them in the locked
condition and in strict alignment with respect to the processing
stations. The above-described array 380 of the variable orifices
381 is achieved due to mutual reciprocal movement of the sliding
strips 383a and 383b, one above the other and in opposite
directions.
[0109] In order to provide such movement, the neck-locking
mechanism 317 is provided with pairs of cams 388a and 388b and
springs 389a and 389b, wherein each pair exerts pressure on the
sliding strip 383a or 383b to urge them in mutually opposite
directions (FIG. 6f). Each sliding strip is provided with a
rectangular opening 390 and a semicircular cutoff 391 arranged on
mutually opposite ends of the strip (FIGS. 6h, k). Each platform
384a and 384b is provided with guides 392 for sliding strips 383a
and 383b and slots 393 for jamming the ends of the semicircular
springs 389 (FIG. 6l). Top and bottom platforms 384a and 384b are
also provided at both ends with two bearing holes 394a and 394b
(for the top platform 384a) and two bearing holes 395a and 395b
(for the bottom platform) 384b (FIG. 6f). The guides 392 restrain
reciprocating movements of the sliding strips 383a and 383b in the
respective platforms 384a and 384b (FIG. 6l). The jamming slots 393
constrain the semicircular spring 389 which is squeezed or expanded
in the semicircular cutoff 391. The bearing hole 394a of the top
platform 384a and the bearing hole 395a of the bottom platform 384b
are pierced by the arm 318a of the clamping device 312 (FIG. 6f).
Respective holes 394b and 395b are pierced by the arm 318b. Both
arms 318a and 318b driven by linear actuators 319a and 319b
simultaneously support and move the neck-locking mechanism 317.
They also rotate the cams 388a and 388b secured on these arms and
situated inside the rectangular openings 390 in the top and bottom
sliding strips 383a and 383b, respectively, which are sandwiched
between the top and bottom platforms 384a and 384b.
[0110] The cams 388a and 388b rotated by the arms 318a and 318b
inside the rectangular openings 390 of the respective slides 383a
and 383b push these slides against the semicircular springs 389a
and 389b which are jammed in the slots 393 of the top and bottom
platform 384a and 384b. Rotation of the cams 388a and 388b by the
arms 318a and 318b is controlled by the step motors 397a and 397b
which are connected to the opposite ends of these arms. The
sequence of operation is illustrated in FIG. 6m.
[0111] Operation of the apparatus will now be described with
reference to the various mechanisms.
[0112] 1. Capturing the Containers
[0113] The goal of this operation is to organize the plurality of
the containers 303 roughly distributed on the belt 313 of the
conveyer 314 into the array 302 of prealigned containers 303. This
function is accomplished by means of the clamping device 312 (FIG.
6n). The linear actuators 319a and 319b move the arms 318a and 318b
with the neck-locking mechanism 317 fixed to their ends from the
vacuum chamber 304 toward the containers 303 which are distributed
on the belt 314 of the conveyer 315 for casting the mechanism 317
onto the necks 338 of the containers 303. In order to pass the
necks 338 through the holes of the neck-locking mechanism 317, the
variable orifices 381 of the array 380 must be enlarged. Before
casting, the step motors 397a and 397b rotate respective arms 318a
and 318b in the opposite direction (FIG. 6f). The cams 388a and
388b are hafted on these arms, turn in clockwise and
counterclockwise directions in the rectangular openings 391 of the
sliding strips 383a and 383b, and are fixed in the transversal
position relative to the strips (FIGS. 6h, k). In such a position,
the cams 388a and 388b do not push the strips against the
semicircular springs 389a and 389b, and the springs shift the
strips in the direction away from the central part of strip toward
to the ends. The spring-loaded sliding strips shift with an offset
that provides superposition of the curvilinear parts 386 with a
large curvature of the oval holes 382 in the sliding strips 383a
and 383b to form orifices large enough for penetration by the necks
338 (FIG. 6g). The necks 338 of the containers 303 pass through the
sequence of the holes 385 of the bottom platform 384a and enlarged
orifices 381 of two sliding strips and holes 385 of the top
platform 384b. Although the necks 338 are captured by the
neck-capturing mechanism 317, they are neither coaxial to the holes
385 of the top platform 384a nor to the holes 385 of the bottom
platform 384a.
[0114] 2. Aligning the Containers on the Belt
[0115] The goal of this operation is precise alignment of the
containers 303 and organization thereof into the array 302 in order
to match the array 306 of the coating stations 307 and the nozzles
of the filling machine (not shown). The horizontal linear actuator
363 of the aligning device 361 allows positioning of the aligning
station 365 with respect to the array of holes 385 of the top
platform 384b of the neck-locking mechanism 317 that has already
captured the necks 338 of the containers 303 (FIG. 6p). As a
result, the array 373 of the fingers 374 is positioned above the
necks 338 of the containers 303. The vertical linear actuator 364
pulls down the aligning station 365 with the air manifold 370 so
that pipettes 376 enter the holes of the mechanism 317 (FIG. 6q)
and immerse into the throats 378 of the necks 338 of the containers
303 positioned on the belt 314. The fingers 374, which are
positioned coaxially to the aforementioned holes, move down through
the necks 338 of the containers 303 to contact the rubber hut 377
with the lips 337. The compressor 371 supplies compressed air
through the manifold 370 to the pipettes 376 immersed in the
throats 378, whereby the pipettes are inflated and assume the shape
of balls 379 (FIG. 6e). The balls 379 which are expanded in the
throat 378 act as self-aligning universal joints that shift the
necks 338 of the containers 303 from the random position to the
position coaxial to the axis of the finger 374 against the
respective holes of the neck-locking mechanism 317. The balls 379
that fill the space 378 inside the necks 338 organize the
containers 303 in the array 302 in accordance with positions of the
fingers 374 in the array 373. Parallel shift of the containers 303
by the expanded balls 379 is possible if the rubber huts 377
restrain movement of the necks 338 in the vertical direction. The
flexible rubber huts 377 cover the lips 337 of the necks 338 and
lightly press the containers 303 against the surface 313 of the
belt 314 of the conveyer 315. The above operation provides strict
alignment of the containers in the array 302 (FIG. 6q) with respect
to the array 306 of the coating stations 307 (FIG. 6n).
[0116] 3. Fixing the Containers in the Aligned Position
[0117] The goal of this operation is to secure the array 302 of the
containers 303 in properly aligned positions relative to the
respective coating stations 307 (FIG. 6n). Since the array 302 of
the containers 303 is already captured by the neck-locking
mechanism 317 and aligned by the balls 379, latching of the necks
338 is achieved by using the aforementioned orifices 381 of
variable geometry. The step motors 397a and 397b turn the arms 318a
and 318b and the respective cams 388a and 388b, thus rotating these
cams simultaneously in the clockwise and counterclockwise
directions, respectively (FIG. 6f).
[0118] Turning the cams 388a and 388b by 90 degrees (FIG. 6g) locks
the necks 338 of the containers 303 of the array 302. Being
positioned in the direction of two-dimensionality of the sliding
strips 383a and 383b, the cams 388a and 388b push these sliding
strips against the respective semicircular springs 389a and 389b,
with the ends nipped in the slots 393 of the platforms. These
springs 389a and 389b are squeezed uniformly due to the surface of
the semicircular cut-offs 391a and 391b at the end of each sliding
strip 383a and 383b. Each neck 338 of each container 303 can be
latched into the grip organized by the opposite narrowing edges 387
of the oval holes 382 of the top and bottom sliding strips 383a and
383b (FIG. 6h,k). The pairs of cams 388a and springs 389a as well
as of cams 388b and springs 389b develop mutually opposite forces
that hold the necks 338 of the containers 303 in the array 302 at
precise intervals (FIG. 6g).
[0119] 4. Removing the Aligning Device
[0120] After alignment, the aligning device 361 becomes an obstacle
on the way to the coating process. The goal of this operation is to
clean the space for delivery of the clamping device 312 with the
array 302 of the containers 303 which are locked in the aligned
position by the neck-locking mechanism 317 for coating the inner
walls of these containers with the barrier layer of silicon
dioxide.
[0121] After accomplishing its functions, the aligning device 361
should be removed. In order to remove the aligning device, the
array 373 of the fingers 374 is extracted from the necks 338 of the
containers 303, and the aligning station 365 is shifted to provide
room for movement of the clamping device 312 (FIG. 6p).
[0122] The vent 372 is turned on in order to drain the compressed
air from the manifold 370 and balls 379. This deflates the fingers
374 back to the form of the pipettes 376 (FIG. 6e). The vertical
linear actuator 364 elevates the slide 366 with the attached
horizontal linear actuator 363 and the aligning station 365, thus
extracting the pipettes 376 from the necks 338. The horizontal
linear actuator 363 shifts the aligning station 365 from the pass
of the clamping device 312 (FIG. 6p). 5. Assembling the Array of
PECVD Chambers
[0123] The goal of this operation is rapid assembling of the array
of individual PECVD process chambers 328 in the vacuum chamber 304,
where each process chamber is comprised of a container 303, a
transversal antenna 325 immersed in this container, an antenna
holder 326, and a gas supply tube 327 (FIG. 6a). In other words,
the process chamber is formed from the coating station when a
container is inserted into this station. The difference between the
process chambers and the coating stations is that the process
chambers include containers 303a through 303n. It is understood
that these chambers 328 are supposed to be reliably sealed and
evacuated before filling with process gas. A sealed vacuum chamber
304 with the PECVD process chambers 328 is shown in FIGS. 6a and
6r.
[0124] Air must be evacuated from the vacuum chamber 304 through
the valve 322, and in order to avoid parasitic discharges inside
this chamber 304, pressure in the chamber 304, which is controlled
by the Baratron 323, must be much lower than inside the individual
PECVD chamber 328. Another reason for such evacuation is to prevent
collapse of the containers 303 if the pressure in the vacuum
chamber 304 exceeds the pressure inside the separated PECVD
chambers.
[0125] Prior to sealing of the containers, the arms 318a and 318b
with the clamping device 312 carrying the array 302 of the
containers 303 is pulled up by the linear actuators 319a and 319b
into the vacuum chamber 304 with the open door 309 (FIG. 6c). The
clamping device 312 continues movement inside the chamber and hafts
the containers 303 onto the antennas holders 326 of the coating
stations 307 to contact the lips 337 of the necks 338 of the
containers 303 with the surface 336 of the antenna holder 316 in
order to form an array of sealed individual PECVD chambers 328
(FIG. 6a).
[0126] Such double evacuation of air prior to the coating operation
is carried out as follows: [0127] (a) movement of the clamping
device 312 brings the lips 337 of the necks 338 of the containers
303 into contact with the surface 336 of the antenna holder 326;
[0128] (b) the door 309 of the vacuum chamber 304 is closed by the
arms 310a and 310b driven by the linear actuators 311a and 311b;
[0129] (c) after closing the door 309, air is evacuated from the
vacuum chamber 304 by the pump 321 through the valve 322; and
[0130] (d) air is evacuated also from the inner volume 337 of the
array of separated PECVD chambers 328 by the pump 350 through the
oblique holes 353 of the antenna holder 326 and the manifold
352.
[0131] 6. Coating Containers With Silicon Dioxide Barrier
Layers
[0132] The goal of this operation is to deposit silicon dioxide
layers having a thickness in the range of 9 to 10 nm onto the inner
surfaces of the barrier layers of the container by using an ICP
discharge. In order to provide high throughput, coating time must
be within 3 to 4 sec.
[0133] An important component of this process is the transversal RF
antenna 325, of the type described above with reference to FIGS. 1
to 5, which is immersed into the containers 303 for generating an
ICP plasma discharge inside the containers in the vicinity of their
inner walls (FIG. 6d). The plasma should have density up to
10.sup.-13 1/cm.sup.3 and uniformity up to 90%. Plasma having the
above-mentioned characteristics decomposes TEOS into a siloxane
backbone (Si--O--Si), decomposes oxygen into atomic oxygen, and
provides a high rate of deposition of silicon dioxide (up to 2
nm/sec) with the formation of a barrier layer of high quality. A
high temperature of plasma facilitates decomposition of oxygen into
atomic oxygen. Lack of atomic oxygen leads to formation of
microparticles of SiO.sub.2 that can precipitate onto the inner
walls of the containers. Also, it is understandable that at such
temperature, the plastic material of the containers, especially
biodegradable plastic, will melt unless special means are taken for
cooling.
[0134] Each antenna 325 with a gas feeding tube 327 is immersed
into a sealed volume 335, 337 inside the container 303 that is
converted in the individual PECVD chamber 328, which is filled with
a mixture of organosilane and oxygen. This mixture, which is
delivered to the chamber 328 through the nonuniformly distributed
holes 334 of the gas feeding tube 327, is prepared in the mixer 344
where the organosilane (TEOS) is delivered from the vaporizer 345
in the form of vapor (FIG. 6a). Oxygen is delivered to the same
mixer from the oxygen tank 347 through the oxygen flow controller
348. The mixer 344 is communicated with the gas manifold 341 that
distributes the mixture uniformly among the gas feeding tubes 327a
and 327b and accordingly sends the mixture to individual PECVD
chambers 328a through 328n. Partial oxygen pressure, which plays a
crucial role in the plasma chemical reaction of TEOS, is tuned by
the flow controller 348.
[0135] Indirect control of pressure in individual PECVD chambers is
carried out by setting the Baratron 323 by activating the flow
controller 348 and vaporizer 345 and filling each volume of the
chamber 328 with precursor gas. Depending on the size of the
container 303, the pressure is set in the range of 2 to 3 Torr to
activate the RF generator 331. The rate of deposition depends on
plasma density and may drastically improve throughput of the
barrier-coating process. Throughput also depends on RF power and
pressure of process gas in the containers, as well as on other
factors, e.g., fill rate of the container 303 with the precursor
without leakage. In order to prevent deposition of organics on the
inner surface of the container, byproducts of the plasma chemical
reaction, such as water vapor and CO.sub.2, must be rapidly removed
from the container. RF power and gas pressure must be optimized to
find a tradeoff between throughput and quality of the deposited
barrier layer.
[0136] The RF discharge 349 (FIG. 6d) in each container 303 filled
with process gas is generated by the respective transversal antenna
325. The transversal antenna 325 is immersed in this container by
energizing RF power from the RF generator 331 connected to the
antenna terminals 332 and 333 through the matching network 330. The
matching network 330 is tuned to resonance and develops high RF
current on the coils of the antenna 325. RF current that circulates
through the antenna coils generates several magnetic fields
according to the number of coils arranged with uniform angular
intervals around the axis of the container. These magnetic fields
propagate with vectors paraxial to the planes of symmetry of the
rectangular or elliptical coils. The magnetic fields, which are
uniformly distributed and directed to the inner wall of the
container 303, are converted into strong electrical fields that
accelerate the charged particles, break down the gas volume,
especially if argon is added to the gas mixture, and generate
high-density plasma that is uniformly distributed in the vicinity
of the inner wall of the container 303. Heat from the plasma
generated by the ICP discharge 349 decomposes organosilane,
especially TEOS vapor, and breaks off methyl groups of
organosiloxane. The discharge also generates atomic oxygen and
initiates a plasma chemical reaction. This oxygen oxidizes methyl
groups and any other organic groups. Further, oxygen oxidizes the
condensable siloxane backbone (Si--O--Si) resulting from TEOS
decomposition and forms a barrier layer 328 of silicon oxide
(SiO.sub.x) on the interior surface 329 of container 303.
Byproducts of plasma chemical reactions, such as CO.sub.2 and
water, are removed by the pump 350 through the oblique holes 353 of
the antenna holder 326 and the manifold 352.
[0137] Under some conditions of the RF discharge 349, the deposited
silicon dioxide layer 328 formed on the inner wall surface 329 of
the container 303 may have a desired transparency, impermeability
to fluids, and strength of adhesion to the plastic walls.
[0138] Quality of the coating depends on the chemistry of the
barrier layer of SiO.sub.x, where X can be optimized in the range
of 2.3 to 2.7, depending on dilution of organosilane by oxygen.
[0139] 7. Cooling the Inner Walls of the Containers
[0140] The goal of this operation is preserving the shape of the
containers 303 and protecting the structure of the plastic from
degradation during the PECVD process in each PECVD chamber 328,
wherein the wall of the container is exposed to high thermal flux
irradiated by plasma (FIG. 6d).
[0141] Decomposition of biodegradable plastic material under the
effect of thermal shock can be prevented if the shock is short
enough and if it alternates with periods of cooling when the
discharge is interrupted and the flow of cold process gas cools the
inner walls. The threshold of degradation of the plastic material
of the container will not be exceeded and throughput will not be
reduced if the periods of hot plasma generation are shunted and
alternated with periods wherein plasma is sustained in a passive
glow state and at a low level of power consumption.
[0142] The working cycle of the each coating station 307 consists
of a coating period and a noncoating period. The interruption of RF
discharge 349 of high energy consumption during the noncoating
period in the coating station is achieved by using dummy loads 355a
and 355b, which consume a valuable part of this power from the
transversal antenna, which is necessary for providing the plasma
chemical reaction. The dummy loads simultaneously shunt and
suppress the discharges in all containers 303 during the noncoating
period when the PECVD process is interrupted.
[0143] The windows 324a and 324b serve for inductive communication
between the antennas 325 and the dummy loads 355a and 355b, which
absorb a valuable part of the RF power from the transversal
antennas 325 so that the RF discharges 349 in the containers 303 do
not extinguish but merely diminish to a weak glow discharge level,
wherein process gas is not converted into hot plasma. During these
cooling intervals, the process gas cools the inner walls of the
containers 303 and is pumped out without decomposition. The
electromagnetic fields from the antennas 325 are strong enough to
penetrate the windows 324a and 324b and generate strong electric
fields in the volumes of the dummy loads positioned next to the
windows. However, argon pressure inside the dummy loads is high
enough and the electric field in the vicinity of the dummy load is
low enough for breakdown in the argon volume and for generation of
the initial charged particles needed for plasma discharge. In order
to generate the particles crucial for generation of the plasma
discharge, the dummy loads 355a and 355b are provided with an
igniting electrode 357 connected to the pulse generator 358. A
spark from the pin of the igniter 357 charges a certain number of
particles during the high-voltage pulse 359 generated by the pulse
generator 358.
[0144] These charged particles injected in the volume of the dummy
loads pierced by the magnetic field propagated from the antenna
facilitate an electric field that is strong enough for breakdown.
RF discharges generated in these dummy loads 325a and 355b consume
a valuable amount of RF power from the antenna through the
propagated magnetic field. In other words, the gas discharges in
the dummy loads 325a and 355b absorb RF power from the RF generator
331 to sustain plasma in the dummy loads 355a and 355b during
noncoating periods used for cooling the containers 303 with cold
process gas in the coating stations. Although during noncoating
periods the RF generator 331 is working under the same operating
conditions, heating of the walls of the containers 303 is avoided
due to provision of the aforementioned dummy loads. Thus, the RF
discharges 361 of the dummy loads are designed to provide cooling
intervals inside the containers 303. By absorbing a valuable part
of RF power, the dummy loads interrupt heating of the inner walls
of the containers 303 by plasma and allow cooling of biodegradable
plastic by process gas.
[0145] Because of a low potential of ionization, argon is used in
dummy loads as a discharge gas because it is more pliable to
triggering and consuming RF power than is the TEOS-oxygen mixture
in the container 303 with a high potential of oxygen ionization.
The plasma of the argon discharge 361 in the dummy loads 355a and
355b permeated by the magnetic field from the antenna efficiently
absorbs RF power from the antenna. In order to sustain the plasma,
the discharge 349 in the TEOS-oxygen mixture needs to consume a
predetermined RF power that is higher as compared with RF power
needed to sustain plasma in argon. In addition, the discharge in
the TEOS-oxygen mixture weakens much faster than in argon. In other
words, the apparatus and method of the invention make it possible
to reduce energy in the PECVD chamber to the level at which plasma
density is below the threshold of the plasma chemical reaction but
above the threshold at which plasma disappears. This period of the
cycle, during which energy transfers to the dummy loads, can be
used to cool the plastic containers of the coating stations with
cold process gas.
[0146] Argon pressure in the dummy load is adjusted to a level high
enough to interrupt the discharge 361 between the ignition pulses
359 during intervals 360 when charged particles are supposed to be
absent. When the discharges 361 inside the dummy loads collapse,
all RF power is applied to sustain the RF discharges 349 inside the
containers 303 in order to continue deposition of the SiO.sub.2
barrier layer 329 onto the cold inner wall of the container
303.
[0147] The generator 358 that generates a sequence of high-voltage
pulses 359 and initiates the plasma discharges 361a and 361b
divides power between the discharges 361a and 361b and the
discharge 349 in the container 303. During depletion of the
discharge 349, a cold precursor, which is still injected into the
container 303 by the gas tube 327, cools the inner surface of the
container. For self-sustaining, each RF discharge 349 inside the
container 303 having the size of a Coca Cola bottle requires just
100 W of RF power at the pressure level of process gas equal to 2
Torr and at a frequency of 60 MHz. Thus, the discharge 349, which
is depleted during consumption of RF power by the discharge
shunting system 354, can be immediately restored during the
intervals 360 between the high voltage pulses 359. During these
intervals, the igniter 357 does not produce sparks to generate
charged particles. Although the stray magnetic field still
penetrates the dummy load, it is weak enough in argon to generate
an electric field that is capable of causing a breakdown of the
dummy loads. Since consumption of RF power by the dummy loads 355a
and 355b is interrupted, ICP discharges inside the containers 303
can start immediately, taking into account the presence of the
charged particles generated by the glow discharge remaining in the
containers during the cooling period. Thus, the PECVD process
inside the containers 303 continues with reduced thermal shock on
the inner walls. The duration of the pulses generated by the pulse
generator 358 and applied to the igniter 357 can be optimized to
simultaneously maintain the wall of the container 303 at a
temperature lower then the degradation threshold and to maintain a
high rate of deposition.
[0148] 8. Delivery of Array of Containers to Conveyer
[0149] The goal of this operation is to place the array 302 of the
coated containers 303 on the nested part 316 of belt 314 of the
conveyer 315 and to release the containers from the clamping device
312 in the cartridge belt with secured intervals between the
containers (FIG. 6b and FIG. 6c).
[0150] After completion of the coating process, the volume of the
containers 303 and the vacuum chamber 304 are vented. The linear
actuators 311a and 311b drive the door 309 out of the chamber 304,
opening the way for extraction of the array 302 of the coated
containers 303. The linear actuators 319a and 319b move the
clamping device 312, thus disconnecting the lips 337 of the necks
338 from the antenna holder 326. Following this, the actuators
return the array 302 of coated containers 303 which are locked by
the orifices 381 to the nested part 316 of the belt 314 of the
conveyer 315. The linear actuators 319a and 319b also provide a
soft touch to the bottoms of the containers 303 with the nests
316.
[0151] After the containers are placed in the nests of the belt,
the step motor 397a performs a 90-degree clockwise turn, and the
step motor 397b rotates the linear actuators 319a and 319b,
respectively, by 90 degrees in the counterclockwise direction (FIG.
6c and FIG. 6f). Rotation of the cams 388a and 388b connected to
these arms in the transversal direction relative to the strips 383a
and 383b terminates pressure of the cam on the sliding strips 383a
and 383b. Being squeezed by the cams and released from their
pressure, the semicircular springs 389a and 389b push apart the
sliding strips 383a and 383 (FIG. 6g). This movement enlarges the
variable orifices 381 and unlocks the necks 338 of the containers
303, which are latched by the narrowing edges 387 of the holes 382
in the strips.
[0152] The clamping device 312 is lifted by the arms 318a and 318b
which are driven by the linear actuators 319a and 319b, leaving the
array 302 of the containers 303 on the nested part 316 of the belt
314. Then the conveyer 315 delivers the coated containers 303 in
the array order with the interval between the containers equal to
the interval in the array of the filling machine nozzles (not
shown).
[0153] The conveyer 315 delivers a new plurality of containers 303
positioned randomly on the flat part 313 of the belt 314 for
coating, and the linear actuators 319a and 319b drive the arms 318a
and 318b with the clamping device 312 back to the belt 314, 313 in
order to organize a new array 302 of the containers 303 and to
deliver them to the chamber 304 (FIG. 6c).
[0154] The ICP discharge simultaneously with coating of the
containers sterilizes the inner walls of the containers. Thus,
alignment of the containers is not required before exposing them to
the filling machine array of nozzles.
[0155] The invention also allows for a double-coating process of
the containers wherein the transversal antenna generates the inner
RF discharge simultaneously with the outer RF discharge in the
outer volume of the vacuum chamber 304, which is filled with
another organometallic precursor. The outer PECVD process sharing
the same antenna allows application of coatings onto the outer
surfaces of the containers simultaneously with application of
barrier layers onto the inner walls of the same containers.
[0156] For example, the outer UV mirror coating capable of
reflecting UV radiation can be applied onto the outer surfaces of
the containers, especially wine bottles, simultaneously with the
formation of silicon dioxide coatings on the inner surfaces of the
bottles if volume of the vacuum chamber 304 outside the bottles is
filled with titanium-containing gases such as titanium isopropoxide
(Ti(O-i-C.sub.3H.sub.7).sub.4). Amorphous TiO.sub.2 thin films can
be deposited on the outer surfaces of containers by means of the
plasma chemical reaction of titanium isopropoxide with oxygen.
[0157] Referring to coating of the hollow containers, it is
desirable to provide uniform deposition of coating films onto the
inner walls of the containers as well as onto the container
bottoms. Because plasma cannot be uniform in each part of a
container, the gas supply system may compensate for such
nonuniformity by using gas-distribution means. For example, the
three-dimensional view shown in FIG. 7 illustrates the
saddle-shaped ICP transversal antenna assembly 420 of the invention
inside the container, e.g., a bottle 422 and a gas supply tube 424
with a plurality of openings 424a through 424n through which
process gas such as hexamethlydisiloxane (HMDSO) or TEOS in mixture
with oxygen and argon is injected into the bottle 422. By varying
the size of the openings in the longitudinal direction of the gas
supply tube 424, the flow rate can be adjusted through individual
openings and thus provide approximately 90% uniform deposition of
the coating and uniform deposition thickness on the inner surface
from the neck to the bottom of the container. The pitch between the
openings may also vary along the gas supply tube in order to select
the most optimal regimen of deposition in accordance with the
specific geometry of the bottle. In other words, the non-uniform
distribution of the openings 424a through 424n should provide
uniform flow of the precursor gas near the container walls. For
example, the diameter of opening 424a may be smaller than the
diameter of opening 424n. Density of openings in the bottom area
can be higher than density of openings in the neck area.
[0158] Reference numerals 428a and 428b designate terminals of the
antenna winding. FIG. 8 is a view similar to FIG. 7 and illustrates
a modification in which the antenna assembly 520 is additionally
equipped with a plasma ignition-triggering device 521. Parts and
elements of the device in FIG. 8, similar to respective parts and
elements in FIG. 7, are designated by the same reference numerals
as in FIG. 7 but with the addition of 100. For example, the
container is designated by reference numeral 522, the antenna
assembly is designated by reference numeral 520, the gas supply
tube is designated by reference numeral 524, and so forth.
[0159] Further improvement can be made to the aforementioned method
of coating the containers with the application of barrier layers by
means of two discharges, one in the container and another in the
dummy load, and with alternating interception of RF power between
both discharges. However, in some situations the sequence of such
alternating interception can be violated if the sequence is
controlled merely by the pulse generator. More specifically, while
discharge in the dummy load with low potential of ionization of
argon can be triggered relatively fast, discharge in the container
with organosilane and oxygen, both of which have a much higher
potential of ionization, takes a longer time. After interruption of
the discharge in the dummy load, the discharge in the container
needs to be boosted in order to accelerate sustaining of plasma.
According to the invention, such boosting can be achieved by
injecting ions of argon, in addition to process gas, into the
container through a separate tube. An additional device for
performing this function should trigger the ICP discharge inside
the container (FIG. 8). Such triggering device may comprise the
following: (1) a solenoid 521a that is a branch of the antenna
winding 520a joined to the RF generator through the matching
device, and (2) a quartz or ceramic gas supply tube 521b joined to
the reservoir of inert gas, such as argon, krypton, etc., (not
shown) through a flow controller (not shown).
[0160] The antenna holder is provided with an additional opening to
allow the tube 521b to pass through the holder and to be immersed
in the container. A strong electromagnetic field of the
aforementioned solenoid 521a (FIG. 9) develops a capillary
discharge that generates ions of argon inside the tube 521b. Arrow
Ar shows the direction of argon ions emitted from the tube 521b and
used for initiating the main discharge in the mixture of
organosilane and oxygen that is reluctant to sustain the discharges
without additional assistance. Such triggering device reduces the
time needed to sustain the plasma during a short RF power pulse
assigned to maintain the biodegradable plastic at a low
temperature.
[0161] Thus, it has been shown that the invention provides a method
and apparatus for application of barrier coatings onto the inner
surfaces of containers. The apparatus and method of the invention
are suitable for application of silicone dioxide layers onto the
inner surfaces of plastic containers at relatively low
temperatures. Deposition is carried out at high speed and with the
possibility of controlling the material deposition process so that
the proposed technique becomes suitable for mass production with
high throughput. The antenna has a three-dimensional shape tailored
to the specific profile of the inner walls of the treated container
and therefore provides uniform deposition of the coating
material.
[0162] Although the invention is shown and described with reference
to specific embodiments, it is understood that these embodiments
should not be construed as limiting the areas of application of the
invention and that any changes and modifications are possible,
provided these changes and modifications do not depart from the
scope of the attached patent claims. For example, the transversal
antenna windings may have shapes different from those shown in the
drawings, and the winding of the antenna may consist of five or
more coils having different dimensions and configurations. The
antenna is suitable for application of coatings onto inner surfaces
of hollow containers made from different plastic and nonplastic
materials and having cylindrical, spherical, semispherical,
barrel-like, or other shapes. The transversal antenna may have an
adjustable three-dimensional geometry for insertion into a
container through a narrow opening in a folded state and with
subsequent unfolding or expansion inside the container for shifting
the windings closer to the inner walls. The windings of the antenna
of the invention may be arranged around a container for inductively
generating plasma inside the container.
* * * * *